The invention disclosed and claimed herein generally pertains to an arrangement for mounting a rotatable X-ray tube anode which provides compensation for thermal expansion of the anode. More particularly, the invention pertains to an arrangement of such type which compensates for displacement of the anode focal spot with respect to the X-ray tube window and collimator, as a result of the anode thermal expansion. Even more particularly, the invention pertains to such arrangement wherein a portion of the heat causing the anode thermal expansion is employed to provide a counter-displacement of the anode and focal spot, i.e., to urge the anode and focal spot back to the positional relationship between the focal spot and the window and collimator which is required for accurate imaging.
An X-ray tube is the principal component of conventional X-ray equipment and computed tomography (CT) equipment. Such tubes contain a vacuum at 10.sup.-8 to 10.sup.-9 torr, and operate to direct a stream of electrons across very high voltage, such as 100-140KV, from a cathode to a focal spot position on a tungsten anode target. X-rays are produced as electrons strike the tungsten, and are directed toward an X-ray transmissive window or port plate, provided in the tube housing or casing. The X-ray beam passes through the window, and also through the narrow aperture or slit of a collimator spaced apart from the window. The collimated beam is directed through a patient, to provide imaging data at an X-ray detector. However, the conversion efficiencies of such tubes are quite low. Accordingly, considerable heat is generated in the anode as a byproduct of the X-ray generation.
In order to reduce heat concentration in the anode, the tungsten anode target is in the form of an annular track, and the focal spot position is a point along the track. The anode is rotatably mounted on bearings, and rotated at high speeds. By means of such arrangement, the electron stream from the cathode is continuously presented with a new and cooler surface. Nevertheless, in a high performance X-ray tube, the surface of the anode may reach temperatures in excess of 2500.degree. centigrade, and areas of the anode outside the immediate target surface may rise to temperatures in excess of 1000.degree. centigrade. The heating of the anode causes growth or expansion of the anode materials. Such expansion, in turn, results in axial motion of the focal spot, i.e., movement or displacement of the focal spot from its intended or ideal position with respect to the tube axis, the X-ray transmissive window, and most importantly the collimator aperture. The aperture is very narrow, such as less than one millimeter. Thus, even comparatively slight axial shifting of the focal spot causes the X-ray beam to pass through the collimator aperture at a position which is off-set from the correct position therefor. Accordingly, as described hereinafter in further detail, the collimated beam will strike the X-ray detector at an off-set position. This is very undesirable, particularly in a CT system, since it leads to serious image quality problems.
In the past, X-ray tube designs have generally done nothing to compensate for anode thermal growth. In order to correct image quality problems resulting therefrom, it has been necessary to make modifications in other components of the CT system. It is anticipated that future developments in CT will place even stricter constraints on axial movement of the focal spot.